Internal combustion engines operated under lean conditions will hereinafter also be referred to as lean burn engines. They are operated using a lean air/fuel mixture. Their exhaust gas therefore comprises not only the usual pollutants carbon monoxide (CO), nitrogen oxides (NOx) and unburnt hydrocarbons (HC) and particles (PM) but also a high proportion of up to 15% by volume of oxygen, so that the exhaust gas has a net oxidizing action. For this reason, the exhaust gas purification methods using three-way catalysts which are customary for stoichiometrically operated internal combustion engines cannot be employed. In particular, the conversion of the nitrogen oxides into nitrogen in the oxidizing exhaust gas atmosphere presents considerable difficulties.
The main components of the nitrogen oxides in the exhaust gas of lean burn engines are nitrogen monoxide (NO) and nitrogen dioxide (NO2), with nitrogen monoxide being the predominant component. Depending on the operating conditions of the internal combustion engine, the proportion of nitrogen monoxide is from 60 to 95% by volume of the total nitrogen oxides.
For the reduction of nitrogen oxides in oxidizing exhaust gases, the method of selective catalytic reduction (SCR) has been known for a long time. Here, ammonia is added as reducing agent to the exhaust gas and this gas mixture is then passed over a catalyst for the selective catalytic reduction (SCR catalyst). The nitrogen oxides are selectively reacted with the ammonia over the SCR catalyst to form nitrogen and water. This method is now used commercially in the purification of offgases from power stations. Typical SCR catalysts comprise, for example, solid state acids from the system TiO2/WO3/MoO3/V2O5/SiO2/SO3 as catalytically active components. Other SCR catalysts are based on acid-resistant zeolites exchanged with transition metals, for example delaminated Y zeolites, mordenite, silicalite or ZSM-5. The working temperature of these catalysts is from about 300 to 500° C.
Owing to the need to add a reducing agent to the exhaust gas, the SCR method is very complicated for use in mobile applications. For this reason, the NOx-storage technology has been developed as an alternative to the SCR method. Here, the nitrogen oxides present in the lean exhaust gas are temporarily stored in the form of nitrates on a nitrogen oxide storage catalyst. After the storage capacity of the storage catalyst is exhausted, it has to be regenerated. For this purpose, the internal combustion engine is briefly operated using a rich air/fuel mixture, i.e. more fuel than can be completely burnt by means of the combustion air is introduced into the air/fuel mixture, so that the exhaust gas is rich. It thus contains unburnt hydrocarbons. The stored nitrates are decomposed into nitrogen oxides in the rich exhaust gas and react with the unburnt hydrocarbons present in the rich exhaust gas as reducing agent to form nitrogen and water.
To store the nitrogen oxides in the form of nitrates, nitrogen oxide storage catalysts comprise basic components such as oxides of the alkali metals and the alkaline earth metals, and also of rare earth metals such as cerium oxide and lanthanum oxide. Preference is given to using barium oxide and strontium oxide. In addition, the nitrogen oxide storage catalysts further comprise catalytically active noble metals, usually platinum. The task of these noble metals is to oxidize the nitrogen monoxide which is predominantly present in the exhaust gas to nitrogen dioxide. Only this is able to react, in the presence of the water vapor in the exhaust gas, with the storage components to form nitrates. During the regeneration of the storage catalyst, the desorbed nitrogen oxides are reduced to nitrogen and water over the catalytically active noble metals.
Cyclic operation of an engine with alternate lean and rich air/fuel mixtures is necessary for the use of nitrogen oxide storage technology for the purification of the exhaust gases of lean burn engines. Here, lean operation is the normal mode of operation of the lean burn engine. During this phase of operation, the nitrogen oxides in the exhaust gas are stored by the storage catalyst (storage phase). During rich operation, the nitrogen oxides are desorbed again and reacted (desorption phase). The storage phase usually lasts for from 1 to 2 minutes, while the desorption phase requires only a brief period of from 5 to 20 seconds.
To improve the two methods described when applied to purification of the exhaust gases of lean bun engines, various solutions have become known. The European patent publication EP 0 733 354 A1 describes a selective catalytic reduction method in which the ammonia required for the reduction is generated from the nitrogen oxides present in the exhaust gas. For this purpose, the exhaust gas purification unit contains a three-way catalyst and a downstream SCR catalyst. The lean burn engine is, as in the case of the nitrogen oxide storage technology, operated alternately with rich and lean air/fuel mixtures. During operation with a rich air/fuel mixture, ammonia is formed from the nitrogen oxides present in the exhaust gas over the three-way catalyst. During this phase, the ammonia is stored on the downstream SCR catalyst. During operation with a lean air/fuel mixture, the nitrogen oxides pass the three-catalyst and are converted selectively into nitrogen and water over the SCR catalyst using the previously stored ammonia. In the method described by EP 0 733 354 A1. the ammonia is thus generated on board the vehicle and does not have to be introduced separately into the exhaust gas.
The German patent publication DE 198 20 828 A1 describes an improvement of the method of EP 0 733 354 A1. For this purpose, a nitrogen oxide storage catalyst is installed upstream of the three-way catalyst. During the lean phases, the nitrogen oxide storage catalyst stores the major part of the nitrogen oxides present in the exhaust gas. During the rich phase, the nitrogen oxides are desorbed and partly converted into ammonia by the three-way catalyst. The exhaust gas purification apparatus described by DE 198 20 828 A1 is said to give an improved degree of conversion of the nitrogen oxides compared to the apparatus of EP 0 733 354 A1.